Mode-Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers.

Exciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost the device efficiency. However, structural diversity of polymers and broad signals from typical electronic spectroscopy have had limits to reveal the interplay between local structures and exciton delocalization. To tackle these problems, here, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode-specific kinetics in the S 1 Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.